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Abstract

The study of texture in thin films is one of the possibilities to obtain information on the fundamental physical processes which govern the thin film growth. Near the substrate, the crystalline orientation of the coating layer will usually be influenced by the surface state of the substrate (grain orientation and dimensions, roughness etc.). However, during the film deposition, the texture of the coating can evolve. This evolution is a particular interesting phenomenon in order to unravel the atomic movements occurring during the deposition. To gain an overall picture for the evolution of texture and development of microstructure with film thickness and substrate material in nanostructured nitride films, a variety of analytical x-ray diffraction (XRD) techniques like θ–2θ scan, pole figure, and residual stress by sin2ψ method were utilized. The coatings of Ti1-xAlxN from different target compositions were deposited onto various substrates (WC-Co, glass, Si(100)). Based on the results obtained, Ti0.5Al0.5N films on WC-Co and glass reveal that surface energy minimization at low thickness leads to the development of (002) orientation. On the other hand, the competitive growth promotes the growth of (111) planes parallel to film surface at higher thickness. However, despite the prediction of growth models, the (002) grains are not completely overgrown by (111) grains at higher thickness. Rather, the (002) grains still constitute the surface, but are tilted away from the substrate normal showing substantial in-plane alignment to allow the (111) planes remain parallel to film surface. Conversely, films on Si had a dominant (002) texture nearly parallel to the surface for all samples, not changing with thickness. This indicates that the surface and interface energy anisotropies provide the driving force for texture development. For films on WC-Co, the stress along (002) which was compressive at low thickness decreases with gradual tilt of (002) and changes to tensile at higher thickness. Tilting of (002) is to minimize the overall energy of the system as the (111) planes develop with thickness store very high compressive stress. On the contrary, the stress state in (002) grains on Si remained compressive through the film thickness. Morphological and roughness observation by SEM and AFM reveal that films on WC-Co and glass display higher roughness than on Si. This confirms that the development of <111> orientation is accompanied by an increase in surface roughness of the film. In contrast, cross- sectional TEM observations reveal a smooth surface region, and the SAED patterns and HR-TEM image support the crystallographic results regarding the texture formation for films on Si. Film hardness was measured by nanoindentation and a correlation between (111) texture, stress, hardness and the type of substrate is obtained. However, for Ti0.67Al0.33N films, the texture growth mechanism is similar to Ti0.5Al0.5N, but the final dominant orientation at higher thickness is (113) and not (111) as expected. Using θ–2θ scans and pole figures, we establish that the preferential orientation changes with film thickness from low index (002) across (111) to high index (113) plane. We have found that to minimize the total energy of the system, the orientation of the growing film switches from (111) to (113) with preferentially less density that causes a decrease in hardness and stress formed in the (111)-oriented grains. Different mechanisms which could explain the crossover of (002), (111), and (113) orientations through film thickness are discussed. Development of the (113) orientation is examined with respect to stress, morphology and mechanical properties. In order to obtain further information on the crystallographic accommodation and to resolve whether the incident flux angle α plays any role in this off-axis texture formation, we have deposited series of Ti0.5Al0.5N films under various incident angles from 0° to 60° on Si(100) substrates at room temperature. We show that both in-plane and out-of-plane crystallographic orientations respond strongly to the deposition angle. For α = 0°, the pole figures display a (111) and (002) mixed out-of-plane orientation with random in-plane alignment. In contrast, under oblique angle deposition (OAD), inclined textures are observed with the (111) direction moving toward the incident flux direction and the (002) moving away, showing substantial in- plane alignment. This observation suggests that TiAlN crystals prefer to grow with the (002) direction perpendicular to the substrate while maintaining the minimization of the surface free energy by maximizing the (111) surface area toward the incident flux. The in-plane texture, which is randomly oriented at normal incidence, gives rise to two preferred orientations under oblique angles – one along the direction of flux and other away from the deposition source. The biaxial texture results from a competition among texture mechanisms related to surface mobilities of adatoms, geometrical and directional effects. The surface and cross-section of the films were observed by SEM. OAD films develop a kind of smooth tiles of a roof structure, with no faceted crystallites. The columns of these films were tilted toward the direction of incident flux. The dependence of (111) texture tilt angle and column angle β on the incidence flux angle α is evaluated using four well-known models. Transmission electron microscopy (TEM) study reveals a voided, intercolumnar structure with oblique growth toward the flux direction. The selected area diffraction pattern (SAED) pattern supports the pole figure observations. Measurements of the nanoindentation test were performed in order to discuss the change of mechanical properties as a function of incident flux angle. Furthermore, when the substrate temperature is elevated from room temperature to 400°C and 650°C, due to the enhanced mobility of the adatoms, the surface diffusion length overrules the self- shadowing effects in traditional OAD at ambient temperature. The mobility affects the angle of columns, off-axis angles of (111) and (002) texture as well as the microstructure and mechanical properties of the coatings. We also demonstrate that the biaxial alignment and inclination of columns is independent of film composition.

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